Analysis of Two-Phase Flow within Porous Structures of Water Electrolysis Cells and Structural Correlation Evaluation
Authors: Omoto Shinnosuke , Yano Takeru , Inoue Gen 
Published: December 2024

In recent years, issues such as environmental pollution and global warming caused using fossil fuels have become increasingly serious, prompting attention to hydrogen derived from renewable energy sources as an energy source. Among the various hydrogen production technologies, attention has recently been drawn to Proton Exchange Membrane Water Electrolyzers (PEMWEs) utilizing solid polymer electrolyte membranes. The advantages of these PEMWEs include rapid response to irregular power supply from renewable energy sources, low environmental impact with no byproducts such as greenhouse gases, and the ability to select from various usage scenarios and operating environments due to their compact and lightweight design. In PEMWEs, a porous transport layer (PTL) is inserted on the catalyst layer to facilitate the removal of generated bubbles. With the improvement in the performance of solid polymer membranes, an increase in concentration overpotential attributed to gas-liquid transport in the PTL has been reported[1]. Accurately analyzing gas-liquid behavior within the porous structure poses challenges. In this study, we investigated the causal relationship between the three-dimensional structure of the porous material and gas-liquid transport properties using computational methods.

To simulate gas-liquid transport within the porous media, a Pore Network Model (PNM) is employed. This model defines the three-dimensional spaces within the porous structure as 'pores' and the connections between pores as 'throats,' solving for the transient mass balance between pores. Flow velocity within the porous media follows principles like Darcy's law, where flux is driven by pressure differences, but also must consider the driving force due to capillary pressure owing to the small-scale nature. In the case of water electrolysis in PEM, the flow rates vary with the current density applied to the device. Generated bubbles progress towards liquid channels within the porous media. It's expected that structures impeding bubble progression would degrade electrolysis performance.

Gas-liquid transport is strongly influenced by the pore size within the porous material. Within the porous structure, as the pore size increases, the capillary pressure required for progression decreases, making transport easier. However, larger pores also tend to trap bubbles within the porous structure, leading to an increase in concentration overpotential. In PEMWEs, various porous medias are being utilized; however, only quantitative evaluations have been conducted[2][3]. This study aims to elucidate the relationship between pore size and pore size distribution and their impact on gas-liquid transport performance, with a focus on non-woven fabric and particle types as the main research subjects.

We confirmed that the three-dimensional structure inherent in each design affects gas-liquid transport properties. In the future, in order to validate the computational accuracy, it is necessary to conduct validation through experimental measurements in addition to calculations.

Acknowledgment

This study was supported by Center of Advanced Instrumental Analysis, Kyushu University.

Reference

[1] B. Han et al., Int. J. Hydrog. Energy, 42, 4478 (2017).

[2] P. Lettenmeier et al., J. Power Sources,
311, 153 (2016).

[3] H.Ito et al., Int. J. Hydrog. Energy, 37, 7418(2012).