Mass-Transfer Rate on a Plane Vertical Cathode with Hydrogen Gas Evolution

Cathodic mass-transfer rate is enhanced by gas bubble evolution. In order to study this phenomena, a sectioned cathode, with insulation between each section, was used. The variations in mass-transfer coefficient, rising velocity of gas bubbles, and thickness of bubble dispersion layer in a vertical direction and due to current density were measured. The void fraction of the bubble dispersion layer was also obtained using these measurements. The measured ionic mass-transfer coefficient was analyzed by applying the additivity rule of micro- and macromixing proposed by Alkire and Lu. It was found that micromixing is caused by the convective flow of electrolyte during bubble growth on the cathode surface, and that the mass-transfer coefficient is proportional to the square root of the evolution rate of hydrogen gas bubbles. On the other hand, macromixing is caused by turbulent natural convection induced by the density difference between the electrolyte on the cathode surface in which hydrogen gas bubbles are dispersed and the bulk electrolyte. The measured mass-transfer coefficient due to macromixing expressed in terms of the Sherwood number was correlated to the Rayleigh number using the following expression Sh_x = 1.72 × 10⁻² Ra_x ^1/2.