Impact of electrode-normal magnetic field on oxygen bubbles generated by alkaline water electrolysis at low current densities

Abstract

The effect of magnetohydrodynamic (MHD) convection on the behavior of oxygen bubbles generated by alkaline water electrolysis is investigated in this paper. In order to avoid mutual obstruction of bubbles and get clear images, a special electrolytic cell with a wire anode is designed to conduct the experiments. The current densities in our experiments are between 0.15 and 0.35 A/cm2, which are relatively low but common values in ordinary industrial electrolyzers. The results show that external magnetic field can reduce the cell voltage slightly and change the distribution of bubble detachment size. The induced MHD convection is found to be able to accelerate the diffusion of gas components in to the bulk electrolyte, resulting in a low comprehensive current efficiency, and further leading to longer growth cycle and larger separation diameter of oxygen bubbles. In addition, different from the “coalesce-and-bounce” detachment pattern, bubbles generated in magnetic field will undergo a sliding motion on the electrode surface during their growth process, which shows a lower coalescence frequency and a smoother detachment. Our work suggests that the driving effect of magnetic field on oxygen bubbles may be positive only at high current densities.