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.

h i g h l i g h t sA mixed-mode accelerated stress test for PEM fuel cells is proposed. High spatial resolution to diagnose PEM degradation is achieved via a segmented fuel cell. The experimental results have been validated using traditional diagnostic methods. a b s t r a c tWith increasing availability of more durable membrane materials for proton exchange membrane fuel cells, there is a need for a more stressful test that combines chemical and mechanical stressors to enable accelerated screening of promising membrane candidates. Equally important is the need for in-situ diagnostic methods with sufficient spatial resolution that can provide insights into how membranes degrade to facilitate the development of durable fuel cell systems. In this article, we report an accelerated membrane stress test and a degradation diagnostic method that satisfy both needs. By applying highamplitude cycles of electrical load to a fuel cell fed with low-RH reactant gases, a wide range of mechanical and chemical stressful conditions can be created within the cell which leads to rapid degradation of a mechanically robust Ion Power™ N111-IP membrane. Using an in-situ shorting/crossover diagnostic method on a segmented fuel cell fixture that provides 100 local current measurements, we are able to monitor the progression and map the degradation modes of shorting, thinning, and crossover leak over the entire membrane. Results from this test method have been validated by conventional metrics of fluoride release rates, physical crossover leak rates, pinhole mapping, and cross-sectional measurements.

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