Modeling of Pore-Scale Capillary-Dominated Flow and Bubble Detachment in PEM Water Electrolyzer Anodes using the Volume of Fluid Method

Gergely Schmidt, Daniel Niblett, Vahid Niasar, Insa Neuweiler

Research output: Contribution to journalArticlepeer-review

Abstract

Setting up fluid dynamics models complements limitedly-available accurate and expensive experiments that investigate the mass transport in PEM water electrolysis. In this work, a first-principle microscale model for anode-side oxygen transport is successfully validated that incorporates the following phenomena: (1) uncertain transport processes in catalyst layers, (2) numerically challenging capillary-dominated two-phase flow in porous transport layers and (3) bubble detachments in flow fields. We developed algorithms for the stochastic generation of geometries and for the coupling of flow and transport processes. The flow model is based on a volume-of-fluid-description of the two-phase Navier-Stokes equations and reproduces experimentally observed velocities of free-moving bubbles within minichannels with a 20 % accuracy, provided that the gas phase’s capillary number is above 2.1 × 10−7. At lower capillary numbers, the used interface capturing method gives erratic velocity fields with excessive spurious currents. At a bubbly to slug flow regime, measured pressure drops are matched within a 30 % margin. The nanoscale mass transport processes within the catalyst layer were simplified to differently distributed flow boundary conditions, all of which yield similar conditions within the porous transport layer. Stable and plausible pressure fields are obtained within porous transport layers at capillary numbers of 5 ×10−7 and above (corresponding to current densities of at least 0.5 A cm−2 for the considered porous transport layer). Gas saturation measurements at (the interface of) the catalyst layer are reproduced by the model with a 20 % deviation and the simulated snap-off of bubbles occur at pore throats that agree with porosimetry results and microfluidic experiments. As a conclusion, the presented model allows explaining and optimizing mass transport processes in channels and porous transport layers at most relevant operating conditions. It can furthermore provide knowledge about boundary conditions for investigating catalyst layer states and related overpotentials.
Original languageEnglish
JournalJournal of the Electrochemical Society
Early online date11 Jun 2024
DOIs
Publication statusE-pub ahead of print - 11 Jun 2024

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