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This paper reports methods to measure porosity and tortuosity of Li-ion battery electrodes. A gas transport resistance measurement method adapted from [Z. Yu, R. N. Carter, J. Power Sour., 195, 1079(2010] is used to characterize an electrode MacMullin number (i.e. the ratio of tortuosity and porosity). Porosity is measured independently, and the tortuosity is obtained from the measured MacMullin number. The measurements were carried out for graphite and Li-metal oxide electrodes taken from an automotive Li-ion battery. Tortuosities of 5.95 ± 0.51 at 28.5% ± 1.3% porosity and 3.74 ± 0.38 at a porosity of 21.5% ± 0.25% were measured for the graphite electrode and the Li metal oxide electrode respectively. Additionally, the tortuosity values for both electrodes were found to be significantly higher than those predicted by the Bruggeman correlation, suggesting that use of the Bruggeman correlation for Li-ion battery electrode modeling is not applicable and tortuosity should be measured for the electrodes of interest.

Data for hydrophilic and hydrophobic pore-size distributions are presented for two gas-diffusion media ͑GDM͒ commonly used in proton exchange membrane fuel cells ͑PEMFCs͒. The data were obtained using two measurement methods, intrusion porosimetry and the method of standard porosimetry ͑MSP͒. The use of multiple working fluids to access hydrophilic and hydrophobic pores is discussed as well as limitations associated with structural changes of the GDM during the tests. The differences in the data between the two methods are discussed for both the carbon-cloth and carbon-paper GDM and it is shown that these differences have significant implications relative to the distribution of hydrophilic and hydrophobic pores that control liquid-water transport. The analysis presented in this work shows that the MSP can lead to a more consistent interpretation of the GDM structure for materials that are compressible.Gas diffusion media ͑GDM͒ play a critical role in proton exchange membrane fuel cells ͑PEMFCs͒ by simultaneously providing several functions that minimize the voltage loss. The GDM must provide ͑i͒ good electronic conductivity to sustain electron flow between the catalyst layer and bipolar plates, ͑ii͒ good thermal conductivity to keep uniform temperature and efficient heat removal, ͑iii͒ mechanical strength to maintain good contact with the catalyst layer and bipolar plates without compressing into flow channels, ͑iv͒ good permeability that allows reactant gas distribution into the reaction sites, and ͑v͒ properties that allow liquid product water removal and prevent accumulation of a liquid-water film on the catalyst.Common GDM consist of porous structures made from carbonfiber paper or cloth, 1,2 and in general, their bulk properties achieve functions i-iv above. However, to achieve function v and adjust the liquid-water removal for various operating conditions, the hydrophobicity of the GDM is increased by treatments with polytetrafluoroethylene ͑PTFE͒ or similar coatings. The hydrophobic treatment is usually obtained by dipping the GDM into an aqueous PTFE suspension. 2,3 Although GDM manufacturers may have proprietary techniques to insure a complete and uniform hydrophobic structure, the structure is typically not specified and may include a combination of hydrophilic and hydrophobic pores. In addition, a microporous layer structure, usually consisting of a mixture of carbon particles with PTFE, is sometimes incorporated on the surface of the substrate to limit the loss of catalyst to the GDM interior and to help wick the liquid water away from the electrodes. These microlayers add to the complexity of the material, making it difficult to understand the key parameters for designing GDM. As a result, GDM formulations are found through rigorous trial-and-error methods, 4-10 and much effort has been committed to develop two-phase models 11-15 and water-measurement techniques 16 in order to understand the effect of liquid water in PEMFC.Two-phase transport through GDM is affected by its morphology and ability to w...

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