Characterizing the Pore Network of Diffusion Media at the Pore Size Scale

Abstract

Water management by gas diffusion media and electrodes is a fundamental aspect of the performance of electrochemical cells including proton exchange membrane (PEM) fuel cells. In these devices, product liquid water blocks part of the available space for reactants flow, leading to mass transport losses and lowering of the performance1. Pores saturation has been observed with different experimental techniques for GDL substrates, however a detailed saturation process at the pore size scale of the microporous layer (MPL) is necessary. Previously, it was suggested that large pore network domains are responsible for an enhanced water evacuation2, however, possible existence of narrow size constrictions could still limit water evacuation. In this work, we detail the benefits of three-dimensional imaging of the pore network at the typical MPL pore size scale, i.e. the sub-micrometre scale, which is achieved by focused ion beam scanning electron microscope (FIB-SEM) nanotomography.Two different MPL designs were analysed: an acetylene black based MPL, named Li100, and a vapor grown carbon fiber (VGCF) based MPL, the latter showing better performance under high relative humidity conditions2, 3. FIB-SEM tomography volumes in Fig. 1a-b were used to observe the relative distribution of larger pore domains and narrower pore constrictions. By removing narrower sized pores from the network and analysing the connections of large pore domains, we show the existence of a characteristic size for pore constrictions below which the pore network is well connected. This characteristic size is depicted by the red line in Fig. 1c-d, which plots the fraction of connected pores as a function of their minimum size. Constrictions size values for the two MPL designs were further validated with mercury intrusion experiments, since constrictions affect all the intrusions by capillary pressure, such as water flow in hydrophobic MPLs.The calculated parameters, which are representative of the connections between large pore domains, open up new perspectives on the prediction of gas diffusion under realistic conditions. The approach allows evaluation of the effects of alternatively removing larger and narrower pore domains, to understand the saturation-driven tortuosity in different MPL structures. J. Shen, L. Xu, H. Chang, Z. Tu, and S. H. Chan, Energy Conversion and Management, 207, 112537 (2020).C. Simon, D. Kartouzian, D. Müller, F. Wilhelm, and H. A. Gasteiger, Journal of The Electrochemical Society, 164, F1697–F1711 (2017).C. Simon, J. Endres, B. Nefzger-Loders, F. Wilhelm, and H. A. Gasteiger, Journal of The Electrochemical Society, 166, F1022–F1035 (2019). Figure 1