Improved experimental method for measuring gas diffusivity through thin porous media

Abstract

Gas diffusion through porous media is critical for the high current density operation of a polymer electrolyte membrane fuel cell, where the electrochemical reaction becomes rate limited by the diffusive flux of reactants. Precise knowledge of the diffusivity through various components in a fuel cell is necessary for accurate modeling and analysis. However, many experimental measurements of diffusivity in literature have high measurement uncertainty. In this study, an improvement to the accuracy of the Loschmidt cell method is presented for measuring the diffusivity through materials with a submillimeter thickness. The diffusivity through various gas diffusion layers (GDLs) is measured, and the relative differences between GDLs are explained using scanning electron microscopy and the method of standard porosimetry. The experimental results from this study and others in current literature are used to develop a generalized correlation for the diffusibility as a function of porosity in the through-plane direction of GDLs.

Effective gas diffusion coefficient in fibrous materials by mesoscopic modeling

“…For instance, gas diffusion in fibrous materials, one of the common occurrences such as mass transport through fibrous screen filters in many filtering processes, and through gas diffusion layer (GDL) in fuel cells, is considered essential in many engineering fields [3][4][5]. A deep understanding of how the fibrous microstructures affect the diffusive flux can lead to significant improvements in product design [6]. Macroscopically a porous medium by definition is a two-phase (solid and air) system, and the gas diffusion efficacy in porous media is usually considered a function of the system porosity e, the internal tortuosity s, and the pore size distributions [6], and has been widely studied via theoretical, experimental and numerical methods [7][8][9][10][11].…”

“…A deep understanding of how the fibrous microstructures affect the diffusive flux can lead to significant improvements in product design [6]. Macroscopically a porous medium by definition is a two-phase (solid and air) system, and the gas diffusion efficacy in porous media is usually considered a function of the system porosity e, the internal tortuosity s, and the pore size distributions [6], and has been widely studied via theoretical, experimental and numerical methods [7][8][9][10][11]. Below we will present a very brief overview of previous work on diffusion in fibrous materials in terms of these different methods respectively.…”

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…40 In the GDM, the characterization of diffusion and governing parameters has been accomplished using various experiments combined with simple limiting-current or Fickian-type diffusion equations to get the values for use in equation 7 or 8. [41][42][43][44] A more direct in-situ measurement of oxygen diffusion resistance has been presented in previous work by Baker et al 38 and Caulk et al 45 that established a method and analysis using limiting-current measurements under dry and oversaturated conditions for use in equation 7 or 8. Although these measurements result in a diffusion resistance while a two-phase condition exists in the PEFC assembly, the degree of saturation and its distribution is unknown; as a result, a direct correlation between saturation and the resulting change in the diffusion resistance is required to validate a simplified saturation model.…”

“…Ex-situ investigations of thermal resistance as a function of saturation have shown that this relationship can be accurately approximated by an effective thermal conductivity. 106,465 In terms of oxygen diffusivity, which is a key quantity impacting performance, in-situ characterization has been investigated with AC impedance spectroscopy, 466 conductivity cells, 41 mercury-intrusion porosimetry, 155,467 X-ray computed tomography (XCT), 42 Loschmidt diffusion cells, 43 and electrochemical techniques. 44 As discussed in the empirical-modeling section, Baker et al 38 and Caulk et al 45 established a method and analysis using limiting-current measurements under dry and oversaturated conditions.…”

An efficient mathematical model for air-breathing PEM fuel cells

“…It is normally represented by Bruggmann's correlation [33]. However, we employ a more accurate correlation that has been specifically developed for the gas diffusion layers by Unsworth et al [34]:…”