This study aims to relate the size, number density, and surface coverage of bubbles adhering to a vertical gas-evolving electrode to the current density and shear rate of alkaline electrolyte flow. A force balance that includes the known forces on the bubbles is formulated. This force balance is made to predict the release radius of the adhered bubbles. Because the electric and Marangoni forces are still an open topic of research, they are approximated using a constant. Also, because the influence of the drag force on bubbles in alkaline water electrolysis is not exactly known, it is approximated using the Stokes drag force with a multiplicative fit factor. An equation for this force balance model that approximates the release radius of the bubbles is formulated. To know the exact value of the shear rate on the electrode, this study uses laminar, fully developed flow. Equations are formulated to relate the fully developed laminar flow to the shear rate. The bubbles adhered to the electrode are visualized using a high-speed camera. The camera films through a PMMA rod that goes through the first electrode, allowing visualization of the bubble-covered surface of the second electrode. A long stainless steel rectangular duct develops the Laminar flow before it arrives at the electrode. The experiment is to relate the bubble size, bubble number density, and surface coverage to the shear rate due to forced flow and the current density. Increasing the shear rate has the biggest effect on the largest bubbles adhered to the surface. The maximal radius reduces, while, for the range in shear rate used in this study, the median stays almost constant. The bigger bubbles will be replaced by smaller bubbles. This will reduce the surface coverage and increase the bubble number density. Increasing the current density, for the range in current density used in this study, means an increase in bubble interactions. This means an increase in bubble coalescence, collisions between bubbles, large bubbles being pushed off the surface by smaller ones behind them, and the increased shear rate from natural convection caused by moving bubbles. These increased bubble interactions will result in a reduction in bubble sizes, surface coverage and an increase in bubble number density. The force balance model is related to the maximal radius of the bubbles. The constant of the electric and Marangoni forces is negative at the anode and positive at the cathode, which is expected because the forces should be attractive at the anode and repulsive at the cathode. The multiplicative fit factor of the drag force due to forced flow indicates a higher force than the Stokes drag force because the flow is not perfectly homogeneous and will flow through other bubbles, which can accelerate the local flow velocities. Also, bubbles will collide at a higher speed with increased forced flow. The multiplicative fit factor of the drag force due to natural convection indicates a higher force than the Stokes drag force because of the increased bubble interactions with increasing current density. Both the higher multiplicative fit factors are observed at the anode and cathode.

This study aims to relate the size, number density, and surface coverage of bubbles adhering to a vertical gas-evolving electrode to the current density and shear rate of alkaline electrolyte flow.
A force balance that includes the known forces on the bubbles is formulated. This force balance is made to predict the release radius of the adhered bubbles. Because the electric and Marangoni forces are still an open topic of research, they are approximated using a constant. Also, because the influence of the drag force on bubbles in alkaline water electrolysis is not exactly known, it is approximated using the Stokes drag force with a multiplicative fit factor. An equation for this force balance model that approximates the release radius of the bubbles is formulated. To know the exact value of the shear rate on the electrode, this study uses laminar, fully developed flow. Equations are formulated to relate the fully developed laminar flow to the shear rate.
The bubbles adhered to the electrode are visualized using a high-speed camera. The camera films through a PMMA rod that goes through the first electrode, allowing visualization of the bubble-covered surface of the second electrode. A long stainless steel rectangular duct develops the Laminar flow before it arrives at the electrode.
The experiment is to relate the bubble size, bubble number density, and surface coverage to the shear rate due to forced flow and the current density. Increasing the shear rate has the biggest effect on the largest bubbles adhered to the surface. The maximal radius reduces, while, for the range in shear rate used in this study, the median stays almost constant. The bigger bubbles will be replaced by smaller bubbles. This will reduce the surface coverage and increase the bubble number density. Increasing the current density, for the range in current density used in this study, means an increase in bubble interactions. This means an increase in bubble coalescence, collisions between bubbles, large bubbles being pushed off the surface by smaller ones behind them, and the increased shear rate from natural convection caused by moving bubbles. These increased bubble interactions will result in a reduction in bubble sizes, surface coverage and an increase in bubble number density.
The force balance model is related to the maximal radius of the bubbles. The constant of the electric and Marangoni forces is negative at the anode and positive at the cathode, which is expected because the forces should be attractive at the anode and repulsive at the cathode. The multiplicative fit factor of the drag force due to forced flow indicates a higher force than the Stokes drag force because the flow is not perfectly homogeneous and will flow through other bubbles, which can accelerate the local flow velocities. Also, bubbles will collide at a higher speed with increased forced flow. The multiplicative fit factor of the drag force due to natural convection indicates a higher force than the Stokes drag force because of the increased bubble interactions with increasing current density. Both the higher multiplicative fit factors are observed at the anode and cathode.

In an effort to make water electrolysis more efficient and simple, flow-through electrolyzers eliminate the need for a gas-separating membrane between the electrodes. However, replacing the membrane with a forced flow field brings new challenges and considerations. In this research, the design of a flow-through electrolyzer is optimized for minimum voltage losses due to bubble formation, pressure drop, and electrolytic resistance. These parameters are all influenced by the potassium hydroxide concentration, the inter-electrode gap and the feed flow rate. The effect on the power dissipation of a flow-through electrolyzer is analytically modelled and validated using experiments, where the design variables are varied.

Additionally, the effect on the resistance in the electrolyte of keeping the gasses dissolved in the electrolyte is studied using IV curves of degassed electrolyte. The gasses in the electrolyte were purged with a vacuum chamber. This degassed electrolyte showed a reduction in overpotential of 150 mV at max. However, this reduction in overpotential was outweighed by the energy requirements to degas the electrolyte. Besides degassing, the effect of suppressing bubble formation by different flow rates was investigated. It was found that there was no noticeable reduction in overpotential between the state where bubbles are suppressed and bubbles were formed.

In addition to the effect of keeping the gasses dissolved, an analytical model was constructed to describe the necessary flow rates to mitigate the electrical resistance due to bubbles in the electrolyte. In the analytical model, solubility plays a big role in determining the necessary flow rate, as solubility is related to the emergence of bubbles. Contrarily, experiments showed the effect of solubility was found to be rather low. By varying the gap width, it was shown that the shear rate at the wall is a better indicator for bubble removal and therefore reduction in resistance due to bubbles.

Furthermore, the shape of the discharge channel was changed to promote uniform flow across the electrodes. The uniform flow should make the product removal at every part of the electrolyzer equal, such that there are no stagnant zones where gas accumulation could build up. This new discharge channel shape was analysed using COMSOL Multiphysics by comparing it with a conventional straight discharge channel. The variable discharge channel outperformed the straight discharge channel in creating a uniform flow across the electrodes for Euler numbers bigger than 10, meaning that the inertial forces are negligible compared to the pressure drop. During experiments, the electrolyzer with a variable discharge channel was tested. This electrolyzer configuration did not perform as well as expected from the theory and simulations, having a higher pressure drop and electric resistance than the conventional electrolyzer. The reason why the variable discharge channel performed poorly was inconclusive.

Lastly, the performance of the various variables was evaluated using the total power dissipation as a function of the current density. This took both the pressure drop across the system and the electrical power consumption of the electrolyzer into account. It was found that using a high electrolyte of 6M potassium hydroxide (KOH) together with a small inter-electrode gap gave the lowest energy dissipation per kg produced hydrogen gas. However, increasing the KOH concentration increases the viscosity and thereby the pressure drop. The theoretical optimum for KOH concentration was calculated to be 5M for this flow-through electrolyzer.