Mass transport at gas-evolving electrodes

Direct numerical simulations are utilised to investigate mass-transfer processes at gas-evolving electrodes that experience successive formation and detachment of bubbles. The gas–liquid interface is modelled employing an immersed boundary method. We simulate the growth phase of the bubbles followed by their departure from the electrode surface in order to study the mixing induced by these processes. We find that the growth of the bubbles switches from a diffusion-limited mode at low to moderate fractional bubble coverages of the electrode to a reaction-limited growth dynamics at high coverages. Furthermore, our results indicate that the net transport within the system is governed by the effective buoyancy driving induced by the rising bubbles and that mechanisms commonly subsumed under the term ‘microconvection’ do not significantly affect the mass transport. Consequently, the resulting gas transport for different bubble sizes, current densities and electrode coverages can be collapsed onto one single curve and only depends on an effective Grashof number. The same holds for the mixing of the electrolyte when additionally taking the effect of surface blockage by attached bubbles into account. For the gas transport to the bubble, we find that the relevant Sherwood numbers also collapse onto a single curve when accounting for the driving force of bubble growth, incorporated in an effective Jakob number. Finally, linking the hydrogen transfer rates at the electrode and the bubble interface, an approximate correlation for the gas-evolution efficiency has been established. Taken together, these findings enable us to deduce parametrisations for all response parameters of the systems.</p

Diffusive and convective dissolution of carbon dioxide in a vertical cylindrical cell

The dissolution and subsequent mass transfer of carbon dioxide gas into liquid barriers plays a vital role in many environmental and industrial applications. In this work, we study the downward dissolution and propagation dynamics of CO2 into a vertical water barrier confined to a narrow vertical glass cylinder, using both experiments and direct numerical simulations. Initially, the dissolution of CO2 results in the formation of a CO2-rich water layer, which is denser in comparison to pure water, at the top gas-liquid interface. Continued dissolution of CO2 into the water barrier results in the layer becoming gravitationally unstable, leading to the onset of buoyancy driven convection and, consequently, the shedding of a buoyant plume. By adding sodium fluorescein, a pH-sensitive fluorophore, we directly visualise the dissolution and propagation of the CO2 across the liquid barrier. Tracking the CO2 front propagation in time results in the discovery of two distinct transport regimes, a purely diffusive regime and an enhanced diffusive regime. Using direct numerical simulations, we are able to successfully explain the propagation dynamics of these two transport regimes in this laterally strongly confined geometry, namely by disentangling the contributions of diffusion and convection to the propagation of the CO2 front.Comment: Submitted to Physical Review Fluid

Mass Transport in Multiphase Electrochemical Systems

Bubbles in electrolysis exhibit significant complexities that greatly affect mass transport at gas-evolving electrodes. This renders disentangling the relevant effects through experiments an extremely tedious task. Therefore, in this thesis we utilize numerical simulations to unravel the controlling mechanisms of mass transfer at gas-evolving electrodes. In chapter 1, we combine the in-situ experiments with numerical simulations to study the effect of single-phase natural convection on the growth and dissolution of bubbles adhering to the electrode. Untangling the effect of diffusion and natural convection, we observe that the experimentally measured bubble evolution can only be accurately described once the flow induced by buoyancy forces is taken into account in addition to the diffusive transport. Furthermore, we reveal the effect of design parameters such as bubble spacing and their arrangement in a clustered network on the convective pattern and the resultant bubble dynamics. In chapter 2, we investigate micro- and macro-convection caused by bubble growth and rise in the electrolyte solution. First, we quantify the hydrogen and electrolyte transport at the electrode by defining an effective Grashof number which accounts for buoyancy forces of gas-in-liquid dispersion. Our findings highlights the dominance of two-phase buoyancy-driven convection over other mass transfer mechanisms. Next, we quantify hydrogen transport to the bubble and derive an expression for gas-evolution efficiency, which is key in determining the bubbles evolution and hence the following mass transfer processes in such systems. In chapter 3, we look into the downward dissolution dynamics of carbon dioxide in a cylindrical water barrier employing experiments and simulations. We reveal that convection causes front convolutions and steepens the gradients nearby. This leads to enhanced diffusive flux across the interface and hence faster propagation of the front. Our findings offer broader insight into the secure storage of CO2 in carbon capture and storage technologies