Pore network models are powerful tools to simulate invasion and transport processes in porous media. They are widely applied in the field of geology and the drying of porous media, and have recently also received attention in fuel cell applications. Here we want to describe and discuss how pore network models can be used as a prescriptive tool for future water electrolysis technologies. In detail, we suggest in a first approach a pore network model of drainage for the prediction of the oxygen and water invasion process inside the anodic porous transport layer at high current densities. We neglect wetting liquid films and show that, in this situation, numerous isolated liquid clusters develop when oxygen invades the pore network. In the simulation with narrow pore size distribution, the volumetric ratio of the liquid transporting clusters connected between the catalyst layer and the water supply channel is only around 3% of the total liquid volume contained inside the pore network at the moment when the water supply route through the pore network is interrupted; whereas around 40% of the volume is occupied by the continuous gas phase. The majority of liquid clusters are disconnected from the water supply routes through the pore network if liquid films along the walls of the porous transport layer are disregarded. Moreover, these clusters hinder the countercurrent oxygen transport. A higher ratio of liquid transporting clusters was obtained for greater pore size distribution. Based on the results of pore network drainage simulations, we sketch a new route for the extraction of transport parameters from Monte Carlo simulations, incorporating pore scale flow computations and Darcy flow.

Recently, pore network modelling has been attracting attention in the investigation of electrolysis. This study focuses on a 2D pore network model with the purpose to study the drainage of water by oxygen in anodic porous transport layers (PTL). The oxygen gas produced at the anode catalyst layer by the oxidation of water flows counter currently to the educt through the PTL. When it invades the water-filled pores of the PTL, the liquid is drained from the porous medium. For the pore network model presented here, we assume that this process occurs in distinct steps and applies classical rules of invasion percolation with quasi-static drainage. As the invasion occurs in the capillary-dominated regime, it is dictated by the pore structure and the pore size distribution. Viscous and liquid film flows are neglected and gravity forces are disregarded. The curvature of the two-phase interface within the pores, which essentially dictates the invasion process, is computed from the Young Laplace equation. We show and discuss results from Monte Carlo pore network simulations and compare them qualitatively to microfluidic experiments from literature. The invasion patterns of different types of PTLs, i.e., felt, foam, sintered, are compared with pore network simulations. In addition to this, we study the impact of pore size distribution on the phase patterns of oxygen and water inside the pore network. Based on these results, it can be recommended that pore network modeling is a valuable tool to study the correlation between kinetic losses of water electrolysis processes and current density.

Porous structures are naturally involved in electrochemical processes. The specific architectures of the available porous materials, as well as their physical properties, crucially affect their applications, e.g., their use in fuel cells, batteries, or electrolysers. A key point is the correlation of transport properties (mass, heat, and charges) in the spatially—and in certain cases also temporally—distributed pore structure. In this paper, we use mathematical modeling to investigate the impact of the pore structure on the distribution of wetting and non-wetting phases in porous transport layers used in water electrolysis. We present and discuss the potential of pore network models and an upscaling strategy for the simulation of the saturation of the pore space with liquid and gas, as well as the computation of the relative permeabilities and oxygen dissolution and diffusion. It is studied how a change of structure, i.e., the spatial grading of the pore size distribution and porosity, change the transport properties. Several situations are investigated, including a vertical gradient ranging from small to large pore sizes and vice versa, as well as a dual-porosity network. The simulation results indicate that the specific porous structure has a significant impact on the spatial distribution of species and their respective relative permeabilities. In more detail, it is found that the continuous increase of pore sizes from the catalyst layer side towards the water inlet interface yields the best transport properties among the investigated pore networks. This outcome could be useful for the development of grading strategies, specifically for material optimization for improved transport kinetics in water electrolyser applications and for electrochemical processes in general.

Neutron imaging was used to visualize and study invasion phenomena in fibrous porous transport layers (PTLs) of titanium felt under different flow conditions of gas and liquid phase. The experiments were realized with flow cells that contained a gas and a liquid flow channel separated by PTLs with different thicknesses and pore size distributions. The invasion can be characterized by counter‐current flow of water and air with joint imbibition and drainage processes. The dynamics were visualized with neutron radiography with a local resolution of 6.5 µm and a temporal resolution of 0.1 s. Individual static gas‐liquid distributions were additionally studied by neutron tomography, with a local resolution of 22 µm and an exposure time of 1.5 s per image (projections: 800/360°). It is shown and discussed that the invasion occurred in continuously repeated imbibition/drainage cycles with frequencies depending on the flow conditions and the PTL structure as well. The change of the PTL saturation with air or water appeared almost independent from the specific PTL structure and the breakthrough of the gas phase occurred at almost constant positions.