Apparent internal cell resistance

Other terms used in Eq. 3.1 are the cell operating current I and the internal resistance of the cell Rj and the apparent (measured) cell resistance R. The formula is simplified in terms of only two kinds of polarization being included (the charge transfer polarization and the concentration polarization). The discussion of other types of polarization that also contribute to the cell s resistance is beyond the scope of this book and can be found in the sources quoted in the Further Reading section of this chapter. 

From the results summarized by chart (Fig. 6.9) it is apparent that carbon-based anodes can contribute as much as two-thirds of the internal cell resistance. This is mostly due to the resistive solid electrolyte interphase (SEI) film formation on the... 

The open-cell voltage (OCV) in hydrogen-oxygen SOFCs at a temperature of 900°C has a value of about 0.9 V, the exact value depending on reactant composition. The relation between current density and voltage of an operating cell, Ui versus i, is practically linear that is, the cell s apparent internal resistance / app has a constant value that does not depend on the current density ... 

It thus appears that in real zeolite crystals diffusion over long distances reflects the influence of surface and internal barriers rather than the pore structure of the idealized framework. As a result the apparent intracrystalline diffusivities often show a strong dependence on the length scale of the measurement. Measurements by QENS and neutron spin-echo methods over distances corresponding to a few unit cells often approach the theoretical values derived by MD calculations for an ideal lattice. Similar values are often obtained by PEG NMR when the measurement is made over short distances. Measurements by most macroscopic methods are on the length scale of the crystals, and these tend to yield lower apparent diffusivities as a consequence of the intrusion of surface barriers and internal resistances due to structural defects. 

However, the temperature increase cannot explain more than half of the deviation of the apparent ASR from the true value. Significant variations in the fuel composition due to localised leaks over the cell area can induce internal currents in the ceil, i.e. the parts of the cell in areas with high local Emf are loaded in the fuel cell mode whereas other parts with Emf below the OCV are loaded in the electrolyser mode. When the cell is externally loaded, internal currents decrease with decreasing cell voltage. Temperature and fuel compositions vary accordingly and thus the local internal resistance also varies. 

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