With the current need to shift from fossil fuels to renewable energies, alternatives to internal combustion engines for transportation are needed. For short range transportation, battery electric vehicles powered by lithium-ion batteries (LIBs) have reached a wide acceptance, while for long range transportation or for heavy duty applications (i.e. trucks, ships, trains), fuel cell electric vehicles based on proton exchange membrane fuel cells (PEMFCs) are foreseen to play a major role. Mercury intrusion porosimetry (MIP) can be a powerful technique to characterize battery and fuel cell materials, which can aid materials optimization and lead to solutions for current issues in terms of energy density and durability.
This article is designed to be a technical guide with practical strategies to use mercury intrusion porosimetry for the characterization and subsequent optimization of battery and fuel cell materials. Increasing the volumetric energy density and thus the range of battery electric vehicles is one of the major goals of current R&D efforts. One approach is to compress the battery electrodes into a thinner layer, which among other advantages results in the same energy stored in a reduced volume. However, there are limitations to the compression: one is determined by the densest possible packing of the approximate spherical active material particles (in the μm range) and the other is set by their internal porosity (in the nm range). MIP is a powerful method to quantify and separate the porosities of these different pore size regimes. This guide will highlight how to properly examine electrode porosities and the advantages of characterizing the morphology of the active material powder prior to electrode fabrication. In fuel cell applications, the mass transport of reactants and products is dominated by diffusion processes, which are governed by the porous networks in the electrodes and the so-called gas diffusion medium. Here, MIP is already an established method and differences in total porosity and pore size distribution have been linked with the achieved cell performance. This article illustrates the aspects that have to be considered in order to avoid measurement artifacts.
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With the current need to shift from fossil fuels to renewable energies, alternatives to internal combustion engines for transportation are needed. For short range transportation, battery electric vehicles powered by lithium-ion batteries (LIBs) have reached a wide acceptance, while for long range transportation or for heavy duty applications (i.e. trucks, ships, trains), fuel cell electric vehicles based on proton exchange membrane fuel cells (PEMFCs) are foreseen to play a major role.[1-2] Mercury intrusion porosimetry (MIP) can be a powerful technique to characterize battery and fuel cell materials, which can aid materials optimization and lead to solutions for current issues in terms of energy density and durability.
This article is designed to be a technical guide with practical strategies to use mercury intrusion porosimetry for the characterization and subsequent optimization of battery and fuel cell materials. Increasing the volumetric energy density and thus the range of battery electric vehicles is one of the major goals of current R&D efforts. One approach is to compress the battery electrodes into a thinner layer, which, among other advantages[3], results in the same energy stored in a reduced volume. However, there are limitations to the compression: one is determined by the densest possible packing of the approximate spherical active material particles (in the μm range) and the other is set by their internal porosity (in the nm range). MIP is a powerful method to quantify and separate the porosities of these different pore size regimes. This guide will highlight how to properly examine electrode porosities and the advantages of characterizing the morphology of the active material powder prior to electrode fabrication. In fuel cell applications, the mass transport of reactants and products is dominated by diffusion processes, which are governed by the porous networks in the electrodes and the so-called gas diffusion medium. Here, MIP is already an established method and differences in total porosity and pore size distribution have been linked with the achieved cell performance.[4-5] This article illustrates the aspects that have to be considered in order to avoid measurement artifacts.
Cathode electrodes commonly consist of the cathode active material (CAM) to which small amounts of conductive carbon and a polymer binder are added. Figure 1 shows a sketch of the electrode structure: the large particles in blue represent the cathode active material (CAM) particles, while the small black spheres represent the conductive carbon; the polymer binder is illustrated in gray. While some CAMs do not have any internal porosity, many do have an elaborate porous network inside the particles and thus a significant internal porosity. Therefore, we would like to distinguish between the inter-particular volume between the CAM particles in the electrode structure and the intra-particular volume inside the CAM particles. The sketch in Figure 1 shows these intra-particular pores in a very simplified and enlarged representation. As the intra-particular pore volume is contained in pore size domains that are significantly smaller than the pore size domains that contain the inter-particular pore volume, the two regimes can be clearly separated by mercury intrusion porosimetry, so that a detailed analysis of the porosity values in the two regimes is possible. For a typical LIB cathode, the electrode is composed of the CAM, a carbon black (CB, serving as conductive carbon) additive, and a polymeric binder (in this case PVdF).
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