The influence of flow shear on the bubble size and surface coverage in alkaline water electrolysis

Abstract

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.