Microstructure-property relationships in a gas diffusion layer (GDL) for Polymer Electrolyte Fuel Cells, Part II: pressure-induced water injection and liquid permeability

Abstract

Keywords:Gas diffusion layer (GDL) X-ray tomographic microscopy anisotropy liquid permeability short-range effect (SRE) A B S T R A C TThe performance of polymer electrolyte fuel cells (PEFC) strongly depends on a controlled water management within the porous layers. For this purpose we investigate liquid water transport in a commercial gas diffusion layer (SGL 25BA) on the pore scale. X-ray tomography experiments combined with pressure-induced water injection provide 3D images of the liquid water distribution inside the GDL at incremental pressure steps between 0 and 100 mbar.The breakthrough behavior of the liquid phase is highly anisotropic. In through-plane (tp) direction first bubble points appear at the outlet plane already at 5 mbar and the 'breakthrough' then evolves continuously over an extended pressure range up to > 30 mbar. For in-plane (ip) direction the breakthrough is discontinuous and takes place at 27 mbar. Simulations of the intrusion process reveal that the different breakthrough behaviors are mainly triggered by different ip-and tp-transport distances. Short tp-transport distances through the thin gas diffusion layer (ca. 100 mm) are responsible for the characteristic continuous tp-breakthrough behavior, which is thus attributed to a so-called shortrange effect.Dedicated methods for 3D-image analysis adapted to fibrous GDL microstructures were presented in part I. With these methods we quantify all microstructure characteristics that are relevant for liquid permeability. These characteristics of pore and liquid phases include size distributions of bulges and bottlenecks, connectivity, effective volume fractions, geodesic tortuosity, constrictivity and hydraulic radius. Quantitative relationships are established between these microstructure characteristics and the liquid permeability, which provide a better understanding of the underlying microstructure limitations for injection and liquid transport.For the in-plane direction the liquid permeability is limited to roughly a similar extent by tortuosity, constrictivity and effective volume fraction. In contrast, for through-plane direction relatively low volume fractions of the liquid phase put stronger limitations to the liquid permeability than tortuosity, constrictivity and hydraulic radius.The curves for relative permeability vs. saturation (and vs. capillary pressure, respectively) achieved from 3D-analysis reveal complex but characteristic (reproducible) shapes with concave, linear and convex segments. The shape of these segments can be attributed to distinct microstructure effects. In contrast, the conventional macroscopic descriptions from literature cannot capture these complex shapes and the underlying microstructure effects. Future investigations with different GDL materials are necessary in order to understand whether these complex shapes for the relative permeability represent a general feature of gas diffusion layers or if they are specific to the investigated SGL material.

Quantifying the influence of microstructure on effective conductivity and permeability: Virtual materials testing

“…Based on [22], we discuss two possible approaches, where the hydraulic radius is defined by means of the specific surface area or by a convex combination of characteristic bottleneck sizes and the median of a "continuous phase size" distribution. The latter approach demonstrates again that the size of bottlenecks is an important microstructure characteristic with respect to permeability [6,14,21,23]. We compare our predictive formulas for permeability with the results obtained in [6].…”

Microstructure Reconstruction and Characterization of the Porous GDLs for PEMFC Based on Fibers Orientation Distribution

“…Reconstruction of a heterogeneous material to obtain the microstructure can be performed using different experimental methods. Recently, X‐ray nano/micro‐computed tomography (CT) and focused ion beam/scanning electron microscopy (FIB/SEM) have been widely used to reconstruct the porous medium 3D microstructures. Ostadi et al created GDL‐microporous layer (MPL) assembly through X‐ray nano tomography and FIB/SEM, respectively.…”

Interfacial transport characteristics between heterogeneous porous composite media for effective mass transfer in fuel cells

“… M ( s nw ) represents the empirical Leverett function related to the PTFE content wt% and liquid saturation s nw . The critical radius for the hydrophobic pores r HO can be derived by combining Equations and as follows: where κ = 1 × 10 −12 m 2 for liquid water . In this study, was used to represent the fraction of pores with sizes larger than r HO in GDLs, and it is defined as …”

“…where κ = 1 × 10 −12 m 2 for liquid water. 52 In this study, f r pore >r HO was used to represent the fraction of pores with sizes larger than r HO in GDLs, and it is defined as f r pore >r HO ¼ number of pores with r pore > r HO total pore number (28) Figure 7 presents the relationship between f r>r HO and the nonwetting phase saturation s nw in the GDLs. It should be noted that water is the nonwetting phase in hydrophobic pores.…”