The effect of electrolytically evolved gas bubbles on the thickness of the diffusion layer II

In order to elucidate the mechanism determining the mass transfer at a gas evolving electrode, the thickness s of the Nernst diffusion layer was determined as a function of the current density for both hydrogen and oxygen evolving on horizontal and vertical electrodes in acid as well as in alkaline solutions. Also the diameter of the bubbles and the diameter of the bubble street were determined. It was found that both s and the slope h of the log s/log i curve strongly depend on the nature of the gas evolved and on the nature of the electrolyte and that the position of the electrode has only a small effect on s and on h. At i <10 mA/cm2 the diameter of free bubbles is practically independent of the current density. At i > 30 mA/cm2 coalescence of bubbles occurs very often with exception of the hydrogen bubbles in alkaline solution. From the experimental results it was concluded that the mass transfer at a gas evolving electrode is determined by the flow of solution along the electrode surface, this flow being caused by detaching and rising bubbles and by coalescence of bubbles.