Concentration Impedance in Testing of Solid Oxide Cells Revisited
Authors: Mogensen Mogens Bjerg , Ebbesen Sune Dalgaard , Jensen Søren Højgaard , Sun Xiufu , Hauch Anne , Chen Ming 
Published: February 2020

Concentration impedance in the context of solid oxide cells (SOC) has been discussed in the literature [1-6]. We divide the concentration impedance into diffusion impedance and conversion impedance. Diffusion impedance is defined as impedance due to a reactant concentration gradient caused by a slow (relative to the electrode reaction rate) diffusion through a porous boy of a stagnant gas layer. Conversion impedance is defined as impedance caused by a decrease in an electrode reactant concentration due the conversion of the reactant by electrode reaction.
Mainly the diffusion impedance has been treated in spite of the fact that the conversion impedance often is of greater importance. Actually not all workers within the SOC field recognize the concept of conversion impedance. Other workers claim that the two kinds of concentration impedance cannot be distinguished.
It is correct that the two phenomena will interact, and thus often it may be difficult to separate them, but we have tools that make it possible to distinguish between the two types of impedance.
The diffusion impedance due to diffusion through a porous body, e.g. an electrode support layer, will not be sensitive to reactant gas flow rate, whereas the conversion impedance will be very sensitive to the reactant flow rate. Diffusion impedance results in the simple, clean case in a Warburg element, i.e. a 45 ° slope followed by a relative sharp decrease by decreasing low frequencies down to the x-axis in a Nyquist plot. In contrast to this, clean conversion impedance can always be modeled by a resistor in parallel with an ideal capacitor, which most often has a rather huge value in the order of 0.1 – 1 F cm-2.
The paper and presentation will explain how the two types of concentration impedance may be extracted from impedance spectra in a manner that they are separated. Both simple examples and more difficult cases with very high fuel utilization at high current density, where the interaction between diffusion and conversion impedance is in particular strong, will be given. Special emphasis will be put on the case of plug flow condition, which is the relevant case for practical cells and stacks.
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