The influence of constrictivity on the effective transport properties of porous layers in electrolysis and fuel cells

Abstract

The aim of the present investigation is to define microstructure parameters, which control the effective transport properties in porous materials for energy technology. Recent improvements in 3D-imaging (FIB-nanotomography, synchrotron X-ray tomography) and image analysis (skeletonization and graph analysis, transport simulations) open new possibilities for the study of microstructure effects. In this study, we describe novel procedures for a quantitative analysis of constrictivity, which characterizes the so-called bottleneck effect. In a first experimental part, methodological tests are performed using a porous (La,Sr)CoO 3 material (SOFC cathode). The tests indicate that the proposed procedure for quantitative analysis of constrictivity gives reproducible results even for samples with inhomogeneous microstructures (cracks, gradient of porosity). In the second part, 3D analyses are combined with measurements of ionic conductivity by impedance spectroscopy. The investigations are preformed on membranes of electrolysis cells with porosities between 0.27 and 0.8. Surprisingly, the tortuosities remain nearly constant (1.6) for the entire range of porosity. In contrast, the constrictivities vary strongly and correlate well with the measured transport resistances. Hence, constrictivity represents the dominant microstructure parameter, which controls the effective transport properties in the analysed membrane materials. An empirical relationship is then derived for the calculation of effective transport properties based on phase volume fraction, tortuosity, and constrictivity.

Tortuosity in electrochemical devices: a review of calculation approaches

“…Consequently, the authors differentiated between two types of tortuosity: 12,13 1. That, which is acquired by indirect calculations based on experimental data τexp.…”

Microstructure-property relationships in a gas diffusion layer (GDL) for Polymer Electrolyte Fuel Cells, Part I: effect of compression and anisotropy of dry GDL

“…geometric, geodesic, hydraulic, electric or diffusional tortuosities), as discussed by Clennell [31]. Similarly, the constrictivity (b) strongly depends on the underlying geometrical concepts for the measurement of pore sizes (bulges) and bottleneck sizes [18][19][20]. Hence, in order to establish reliable quantitative relationships between these microstructure characteristics and the effective properties one has to take into account the specific definitions of the microstructure characteristics as well as the measurement concepts used for image analysis.…”

“…(7) and (8) is considered as a general description of the microstructure-property relationship for diffusion and conduction, since it is derived statistically based on a large number of 3D microstructures with very different characteristics. It must be emphasized repeatedly that the relationship only holds for specific definitions of b [18][19][20] and t geodesic [24], which are described in the methods part below. It was also shown in experimental studies with porous ceramic membranes [25] and with cermet-electrodes for solid oxide fuel cells [34] that Eqs.…”

“…In tubes with varying diameters constrictivity is defined as the ratio of cross-sectional area at bottlenecks (A min ) over cross-sectional area at pore bulges (A max ) [19].…”

“…Specific microstructure characteristics can be extracted from the tomography images, which are relevant for the effective transport properties (i.e. porosity, pore size distributions, tortuosity, constrictivity, hydraulic radius) [18][19][20][21][22]. Nowadays it is also possible to use voxel-based 3D-data as input for numerical simulations (e.g.…”