Publication
Hydrogen production through water electrolysis is a clean option for storing energy from renewable sources. Low-cost hydrogen production requires efficient water electrolyzers that can produce pure hydrogen at high production throughputs. Membrane-less electrolyzers are promising technologies for hydrogen production due to their simple design, low ionic resistance, and adaptability to various working conditions. The fluidic flow keeps hydrogen and oxygen bubbles from each other in these electrolyzers. Bubbles' motion in these electrolyzers affects product purity and electrochemical performance. Therefore, a thorough understanding of the two-phase flow is needed to improve the performance of membrane-less electrolyzers. In this thesis, we explore the effects of different parameters on the bubbles' trajectory and present methods to enhance the performance of the membrane-less electrolyzers. In the first step, we study the bubble motion in rectangular microchannels. The results indicate that the Reynolds number, Capillary number, Bubble diameter, and channel aspect ratio determine the bubble lateral equilibrium position in the channel. These four parameters together can be modified to control the final equilibrium position of a bubble in a rectangular channel. In the next step, we study the effect of bubble nucleation and interaction on the bubble cross over in membrane-less electrolyzers. We find that the large bubble detachment from the surface of the electrode and the bubble coalescence are two phenomena that lead to the bubble cross over. This problem can be resolved by increasing the flow velocity in order to detach bubbles at a smaller diameter and remove them faster from the channel. However, the energy loss due to the fluidic flow increases with the flow velocity. We showed that the bubbles become smaller without increasing the flow velocity by adding a surfactant to the electrolyte. A surfactant reduces the bubble detachment size and prevents the bubble coalescence. Furthermore, a surfactant can decrease overpotentials by reducing the residence time of bubbles on the surface of the electrode. We use the results of the two-phase flow studies to design the porous wall electrolyzer as a new design for membrane-less electrolyzers. This design is optimized for high throughput production of hydrogen with low cross over. The porous wall electrolyzer has a significantly smaller cross over compared to membrane-less electrolyzers with parallel electrodes. The porous wall electrolyzer operates at the current density of 300 mA/cm^2 with 0.11±0.05% hydrogen cross over to the oxygen side. Finally, we develop a method for the measurement of fluidic properties. This method uses neural networks and images of two-phase flows to measure flow conditions such as flow rate and fluidic properties such as concentration. This measurement technique can be used for controlling two-phase flows in the membrane-less electrolyzers in order to prevent bubble cross over.