Abstract
ABSTRACTThe tortuosity of a structure plays a vital role in the transport of mass and charge in electrochemical devices. Concentration polarisation losses at high current densities are caused by mass transport limitations and are thus a function of microstructural characteristics. As tortuosity is notoriously difficult to ascertain, a wide and diverse range of methods have been developed to extract the tortuosity of a structure. These methods differ significantly in terms of calculation approach and data preparation techniques. Here, a review of tortuosity calculation procedures applied in the field of electrochemical devices is presented to better understand the resulting values presented in the literature. Visible differences between calculation methods are observed, especially when using porosity–tortuosity relationships and when comparing geometric and flux-based tortuosity calculation approaches.
Highlights
Electrochemical devices, including fuel cells and batteries, will play an increasing role in our lives, as we transition to a low-carbon economy
A review of tortuosity calculation procedures applied in the field of electrochemical devices is presented to better understand the resulting values presented in the literature
It is evident that the tortuosity values of the solid phases are higher compared to those of the pore phase, which is identical to the findings presented by Chen-Wiegart et al [114] using a distance mapping approach
Summary
Electrochemical devices, including fuel cells and batteries, will play an increasing role in our lives, as we transition to a low-carbon economy. As introduced in previous sections, diffusive mass transport and, as such, mass transport limitations are a function of the complex microstructure of the involved porous membrane layers Microstructural parameters, such as tortuosity, are achievable by measuring concentration losses of fuel cells and applying gas diffusion theory. When applying the same calculation approaches to two SOFC samples, the geometrically derived tortuosity values for the pore and YSZ phases were consistently below the diffusion-based tortuosity methods. Large variability in homogeneity of a sample significantly affects the results achieved by the pore centroid method, causing visible fluctuations In this respect, Cooper [118] pointed out that if the analysed characteristic feature becomes small compared to the control volume, the centroid of each 2D plane will tend towards the centre, resulting in a tortuosity of unity which casts doubt on the applicability of this approach. Microstructural inhomogeneities might be the cause of failure mechanisms and material fractures [51]
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