Water management is an important factor for optimizing polymer electrolyte fuel cells (PEFC) under high current density conditions as required for the automotive application. The characteristics of the local liquid saturation of the gas diffusion layer (GDL) is of particular interest. Here we report on the development of in-situ X-ray tomographic microscopy (XTM) with a pixel sizes in the order of 2 μm and sensitivity for carbon and liquid water for the quantitative analysis of liquid water in GDLs. In-situ XTM of PEFC is a major experimental challenge. A complete cell needs to be operated under realistic conditions in the constraint space of the small field of view on the beamline sample stage. Further phase segmentation of the images is required to successfully analyze the quantitative properties of the different phases. For this a workflow, applying differential images between dry and wet structures has been developed. Cells with Toray TGP-H-060 GDLs were analyzed in-situ. Droplets that appear on the GDL surface are connected to a significant water structure inside the GDL. Further the water cluster size distribution in the GDL shows that while small droplets (<100 pl) are numerous, most of the water is contained in few larger clusters.

Sluggish vanadium reaction rates on the porous carbon electrodes typically used in redox flow batteries have prompted research into pretreatment strategies, most notably thermal oxidation, to improve performance. While effective, these approaches have nuanced and complex effects on electrode characteristics hampering the development of explicit structure–function relations that enable quantitative correlation between specific properties and overall electrochemical performance. Here, we seek to resolve these relationships through rigorous analysis of thermally pretreated SGL 29AA carbon paper electrodes using a suite of electrochemical, microscopic, and spectroscopic techniques and culminating in full cell testing. We systematically vary pretreatment temperature, from 400 to 500 °C, while holding pretreatment time constant at 30 h, and evaluate changes in the physical, chemical, and electrochemical properties of the electrodes. We find that several different parameters contribute to observed performance, including hydrophilicity, microstructure, electrochemical surface area, and surface chemistry, and it is important to note that not all of these properties improve with increasing pretreatment temperature. Consequently, while the best overall performance is achieved with a 475 °C pretreatment, this enhancement is achieved from a balance, rather than a maximization, of critical properties. A deeper understanding of the role each property plays in battery performance is the first step toward developing targeted pretreatment strategies that may enable transformative performance improvements.

Operating Polymer Electrolyte Fuel Cells (PEFC) under high current density conditions, causes significant losses related to liquid water saturation in the gas diffusion layer (GDL). The blockage of pores inside the material has a strong influence on its effective gas transport properties. Here we report on the combination of in-situ X-ray tomographic microscopy (XTM) of PEFC and the numerical determination of gas transport properties using Lattice Boltzmann and finite difference methods. The GDL domains (Toray TGP-H-060) of two identical cells, each with 11 mm 2 active area, were analyzed in sections of about 0.3 to 0.8 mm 2 size. Saturation levels between 0.1 and 0.4 were found, with higher saturation under the ribs. The saturated and the non-saturated states of the GDL samples were compared in order to quantify the dependence of gas phase permeability and effective relative diffusivity on liquid water saturation. Both these relative measures were found to follow power relationships of (1 − s) λ , where the exponent λ was approximately 3 for all cases except for the in-plane diffusivity where it was closer to 2.Performance of polymer electrolyte fuel cells (PEFC) at high current densities is limited by mass transport losses associated with the presence of liquid water in the porous structures. In the gas diffusion layers (GDL) this issue is particularly relevant.The porous GDL structure allows collecting current under the flow field channels and provides access for the gases under the ribs. 1 This requires a high permeability and relative diffusivity in the pore space and a high conductivity in the solid. GDLs, made from carbon fibers with diameters in the order of typically 6-8 μm, have porosities around 75% and the internal surface is treated with hydrophobing agents. The formation and transport of liquid water in the GDL is thus governed by its internal structure and surface properties, and the presence of liquid water in the pore space of the GDL influences its gas transport properties. At high current density and/or moderate temperature conditions, the oxygen transport in the cathode GDL is particularly affected, which may lead to significant overvoltages. 2,3The effective relative diffusivity of GDLs has been determined with sulphuric acid filled samples and an electrochemical method. 4,5 LaManna et al. 6 have obtained the effective relative diffusivity in a test cell by inducing mass transfer using a concentration gradient of water vapor. These methods however fail to produce saturation dependent results. Opposite to the experimental methods, effective medium theory was used to describe the diffusion through a porous medium consisting of packed spheres. 7 Later, numerical models yielded similar relations particularly for hydrophobic fibrous materials such as the GDL. Tomadakis and Sotirchos 8 used Monte-Carlo simulations in fiber structures and found that the effective relative diffusivity was strongly dependent on the fiber orientation. Nam and Kaviany 9 extended the work of Tomadakis and Sotirchos 8 and...

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This paper presents an approach to quantify microstructural inhomegeneity in lithium ion battery electrodes over multiple length scales and examines the impact of this microstructural inhomogeneity on electrochemical performance. Commerical graphite anodes are investigated because graphite remains the anode material of choice due to its low cost, mechanical robustness, and suitable electrochemical properties. At the same time, the graphite anode often plays a role in cell degradation and failure, as lithium plating can occur on the graphite anode during charge, when unfavorable microstructure in the graphite electrode leads to a large overpotential. Here, three-dimensional representations of four different commercial anodes obtained with X-ray tomographic microscopy are statistically analyzed to quantify the microstructural inhomogeneity that is commonly present in lithium ion battery electrodes. Electrochemical simulations on the digitalized microstructures are performed to isolate and understand the influence of different types of microstructural inhomogeneity on battery performance. By understanding how distributions in particle size and shape or slurry and electrode processing cause microstructural inhomogeneity and impact performance, it is possible to determine the extent to which homogeneity should be prioritized for specific applications and how homogeneity could be achieved through smart material selection and processing.