Volume of Fluid Modeling of Capillary-Dominated Flow Patterns and Bubble Detachment in PEM Water Electrolyzers

Low-emission hydrogen accounted for less than 1 % of global hydrogen production by 2023, but will have to increase more than 100-fold by 2030 according to the International Energy Agency’s net-zero emission scenarios for 2050. Proton exchange membrane water electrolyzers are particularly suitable to produce hydrogen from renewable energy sources, yet the currently available technological combinations are considerably more expensive than producing hydrogen from fossil fuels (by 65 % to 810 % according to the International Renewable Energy Agency’s 2021 report). To reduce costs, the materials and dynamic operating conditions in electrolyzers must be optimized, amongst other things with regard to low oxygen concentrations (waste product) at the catalysts. We use a first-principle microscale model for oxygen transport to complement experimental optimization efforts, which are generally expensive and limited by measurement accuracies.

The model deploys the volume of fluid method and accounts for (1) uncertain transport processes in the catalyst layer, (2) numerically challenging two-phase at capillary numbers as low as 2.1 · 10^-7 and (3) bubble detachments in channels. The model is validated with respect to flow patterns in microfluidic experiments as well as to pressure drops and bubble velocities within minichannels (30% and 20% match regarding the latter two). The model is numerically stable at operando conditions with at least 0.5 A/cm² current density in a stochastically reproduced porous transport layer. Uncertain catalyst-side solute transport and nucleations are implicitely accounted for, yet their spatial variations are found to negligibly affect the conditions inside the porous transport layer.  Operando gas saturation measurements are locally matched within a 20% margin and are qualitatively matched across the entire porous transport layer.

The simulated bubble detachment in flow field channels occur at pore throats that agree with porosimetry and microfluidic experiments. The gaseous phase pressure fluctuates greatly according to the detachment throat size and the bubble diameter immediately before detachment. The model allows the prediction of nucleation and detachment sites and can be further utilized to optimize porous transport layers as well as to predict boundary conditions when modeling catalyst layers and flow fields.