Electrolytic gas evolution is a fundamental phenomenon occurring in a large number of industrial applications. In these processes gas bubbles are formed at the electrode from a supersaturated solution. Since dissolved gases can change the surface tension, a gas concentration gradient may cause the surface tension to vary locally at the interface of the gas bubble. Surface tension gradients may also form due to temperature gradients generated by ohmic heating of the electrolyte. In both cases, the resulting shear stress imposes a convection in the electrolyte and the gas bubble (Marangoni effect). This phenomenon may influence the entire electrolytic gas evolution process, e.g., by an enhanced mass transfer. In this study, the first evidence of the Marangoni convection near growing hydrogen bubbles, generated by water electrolysis, is provided. Microscopic high speed imaging was applied to study the evolution of single hydrogen bubbles at a microelectrode. The convection near the interface of the growing bubble was measured by using a time-resolved Particle Tracking Velocimetry (PTV) technique. The results indicate a clear correlation between the magnitude of the Marangoni convection and the electric current.

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Water electrolysis was carried out in a 1 M H 2 SO 4 solution under different potentiostatic conditions in the presence of a magnetic field oriented normal to the horizontal microelectrode (100 μm in diameter). The imposed magnetohydrodynamic (MHD) electrolyte flow around the evolving hydrogen bubble was studied to clarify the effect on the detachment of the bubble from the electrode and the mass transfer toward the electrode. Different particle imaging and tracking techniques were applied to measure the three-dimensional flow in the bulk of the cell as well as in close vicinity of the evolving bubble. The periodic bubble growth cycle was analyzed by measurements of the current oscillations and microscopic high-speed imaging. In addition, a numerical study of the flow was conducted to support the experimental results. The results demonstrate that the MHD flow imposes only a small stabilizing force on the bubble. However, the observed secondary flow enhances the mass transfer toward the electrode and may reduce the local supersaturation of dissolved hydrogen. Renewable energy technologies have become increasingly important in view of the worldwide rising CO 2 emissions. The growing application of volatile energy sources such as wind and solar power requires efficient energy storage and distribution systems in order to sustain stability in the electrical grid. Hydrogen shows great potential for long-term energy storage as well as mobile units employing fuel cells as it provides a large energy density. Moreover, high purity hydrogen (and oxygen) can be directly generated from the electricity produced by a wind turbine or solar panel by the electrolysis of water. However, one main challenge for the establishment of water electrolysis on an industrial scale is its relatively low efficiency, typically in the order of 60% for conventional alkaline water electrolyzers. 1 A considerable part of the losses is the result of hydrogen and oxygen gas bubbles evolving at, and sticking to the electrodes' surfaces, thus hindering the formation of new gas bubbles. Furthermore, they reduce the effective electrical conductivity of the electrolyte, which leads to an increased ohmic voltage drop. The accelerated removal of gas bubbles from the electrode's surface and the bulk was studied by many researchers to improve the efficiency of the process.2-6 Forced convection has been found to be able to enhance the detachment of bubbles from the electrode and therefore reduce the fractional bubble coverage significantly.7-12 A relatively simple method that has recently attracted considerable research interests in this context is the application of magnetic fields. The superposition of a magnetic field on the inherent electric field gives rise to Lorentz forces f L = j × B acting as body forces directly on the electrolyte, where j denotes the current density and B is the magnetic induction, respectively. 13 The flow generated by these Lorentz forces is often referred to as the magnetohydrodynamic (MHD) effect.It was shown that stirring with...

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