(354e) Impact of Droplet/Bubble Growth Dynamics on Electrode Limiting Current

Electrochemical devices often involve multiple phases, whether it is liquid-water droplets in hydrogen fuel cells or hydrogen and oxygen bubbles in water electrolyzers. The formation of these phases is directly coupled to the reaction sites within the catalyst layers, thus a complex multi-phase flow problem develops in these devices. In fuel cells, the production of water forms droplets that can retard the flow of reactant gas and make the catalyst inactive, dramatically reducing device performance. The problem is less egregious in electrolyzers, where the production of gas forms bubbles on the electrode surfaces, which eventually grow large enough to detach and exit the catalyst layer and may impact reactant transport to the underlying catalyst surface. In this work, we present a model for the transport of reactant (oxygen or water) through a growing droplet (bubble) of liquid (gas) and examine how the reaction rate or current density at the surface is impacted. The droplet grows primarily through water production at the electrode surface, while the bubble grows via two primary mechanisms, oxygen production at the electrode surface and supersaturation of oxygen in the water surrounding the bubble. Key components also include a force balance for size, transport within and outside the domain including the induced flux at the catalyst interface, and vapor-liquid equilibrium at the surface. The diffusion equation in cylindrical coordinates is used to capture the transport processes. This model provides significant insight into remaining transport inefficiencies in fuel cells and electrolyzers and how they can be mitigated.