Visualization of bubble-induced mass transport at vertical hydrogen evolving electrodes

Gas-evolving electrodes, for example in electrolysis applications, benefit from enhanced mass transport driven by movement of gas bubbles. To maximize this “bubble-induced mass transport”, we need to better understand the underlying processes. Here, we present an optical investigation of microscale mass transport processes at vertical hydrogen evolving electrodes. Using fluorescence lifetime imaging microscopy (FLIM), we show that the concentration boundary layer thickness is mainly reduced by bubble detachment. Combined with velocity profiles around bubbles at a nickel electrode, using micro-particle image velocimetry (μPIV) and an integrated mass transport model, we show that bubble detachment and wake flow reduce the boundary layer thickness with a factor 4-6. Our observations also show that isolated single bubble-events do not exist at vertical electrodes, due to frequent wake flow events from passing bubbles, even at low current densities (5 mA cm-2), which requires combined in-operando measurement techniques to assess mass transport at relevant conditions.