Investigating Mass Transfer Around Spatially-Decoupled Electrolytic Bubbles

Electrolytic bubbles have a profound impact on mass transport in the vicinity of electrodes, greatly influencing the electrolyzer efficiency and cell overpotential. However, high spatio-temporal resolution experimental measurements of concentration fields around electrolytic bubbles, are challenging. In this study, a succession of spatially-decoupled electrolytic bubbles are simulated. The bubbles grow, and departing from a hydrophobic cavity at the center of a ring microelectrode. The gas-liquid interface is modeled using a moving mesh topology. A geometric cutting protocol is developed to handle topology changes during bubble departure. The simulated bubbles show good agreement with the bubble growth dynamics observed in experiments. The bubbles in this spatially-decoupled system outgrow the region of electrolyte that is saturated with dissolved hydrogen. This leaves the apex of the bubble interfaces exposed to an undersaturated region of the electrolyte which leads to an outward flux of hydrogen gas. This is shown to limit the gas evolution efficiency of bubbles despite the fact that they grow at a constant volumetric rate. By analyzing the distribution of the flux of dissolved hydrogen along the bubble interface along with the development of dissolve hydrogen concentration profiles around the bubble, we show that the magnitude of the outward diffusive flux at the apex of the bubble decreases with increasing electrolysis current.