Towards Multiphase Modeling and Simulation of Alkaline Water Electrolysis through Pore-Resolved Foam Electrodes

Abstract

In today’s world, concern is growing about the future of energy. Despite very ambitious international climate goals by 2050, global energy-related carbon dioxide (CO2) emissions keep increasing. In order to tackle this problem, hydrogen (H2) might become a significant part of the solution since it is a way to produce, store, move and use energy in a clean way. However, more than 95% of the actual hydrogen production is made of grey hydrogen, e.g. H2 produced from fossil fuels, which leads to high CO2 emissions [1]. One way to decarbonise this energy vector is to use renewable energies (solar panels, wind turbines, etc.) to produce green hydrogen via water electrolysis.Among the various technologies available to perform water electrolysis, alkaline water electrolysis (AWE) is the most mature one. In general, AWE consists of two planar electrodes separated by a certain distance and operating in a liquid alkaline electrolyte solution (e.g. KOH, potassium hydroxide). Recent studies show that it is possible to significantly increase the performance of such electrolysers by substituting the traditional gap-cell design by a zero-gap cell [2,3]. To do so 3D electrodes such as foams and 3D-printed structures are used, leading to an increase of the surface area and as a result, to a higher hydrogen production rate. These 3D electrodes are incorporated between the planar current distributors at either side of a membrane or diaphragm, reducing the interelectrode distance to the thickness of the separator. In the case of traditional cells, the hydrogen bubbles will tend to evolve only from the surface of the 2D current distributor plates, while for zero-gap cells gases evolve inside the porous structure. The process can be improved even more by forcing the electrolyte flow, favouring bubble removal. Finally, a bi-layer configuration with a catalyst layer presenting high specific surface area near the separator along with a porous transport layer (PTL) allows for a very high hydrogen production in the catalyst and an easy evacuation of the bubbles through the PTL.Recent experimental results in our group validate the enhanced performance of a bi-layer foam configuration where a fine foam acts as the catalyst layer and a coarser one as the PTL. In order to have a better understanding of the electrolyte flow behaviour inside these 3D electrodes, Computational Fluid Dynamics (CFD) was used as a tool to simulate the single-phase flow numerically. The purpose of this is to determine optimal flow parameters in order to favour hydrogen bubble removal while benefiting from the high surface area of the 3D electrodes. Up to now, it appears that this kind of analysis has rarely been addressed in the literature [4].In our specific case, the foams used as the catalyst and the PTL have characteristic pore sizes of 450 µm and 3000 µm, respectively. In order to obtain an accurate geometric representation, the foams were scanned using high-resolution micro-CT (computed tomography). A computational mesh was then obtained by reconstructing the highly detailed scanned data. The incompressible Navier-Stokes equations for the single-phase flow were solved using the Finite Element Method (FEM) implemented in MigFlow software [5]. In Figure 1 we present results from a simulation with the PTL width being three times the width of the catalyst layer, and the inlet velocity 1 m/s. It appears that the electrolyte velocity in the direction perpendicular to the flow is the highest in the area close to the interface between the catalyst and the PTL, meaning that this configuration would help extracting hydrogen bubbles off the catalyst layer, towards the coarser porous region.It is important to note that the results obtained in this case were not considering the gas phase. We are currently working on the development of a multiphase pore-resolved model to see whether the assumption of optimal bubble removal is confirmed for multiphase flow. To this end, a Eulerian-Eulerian two-fluid model must be implemented for the liquid electrolyte and the gaseous bubbles. One key aspect is to correctly define closure relations for the interfacial exchange terms that appear in the momentum equations. Hydrogen bubble formation, growth and escape from the porous electrodes might also be considered. This work is a first step towards the modeling of the whole alkaline water electrolysis process through 3D porous electrodes. Figure 1