The performance limiting processes in SOFC electrodes become accessible by Electrochemical Impedance Spectrocopy (EIS). Generally we analyze the measured impedance spectra by the distribution of relaxation times [1] and a subsequent Complex Nonlinear Least Square approximation to a physically motivated equivalent circuit model [2]. This approach enables us to identify and quantify the contribution of different loss mechanisms in the cell.
In most anode-supported SOFCs the fuel electrode is realized by two porous Ni/YSZ layers, the supporting substrate (AS) with a thickness of 200 … 1000 µm and the anode functional layer (AFL) with a thickness of 5 … 20 µm. The two layers are characterized by specified microstructure properties, which are volume fractions of nickel and YSZ, porosity, tortuosity and triple phase boundary length (fig. 1b). These electrodes, especially the AFL, provide a large number of electrochemically active triple phase boundaries (TPB), where (i) the electrochemical oxidation of hydrogen couples (ii) the electronic conduction in the nickel matrix, (iii) the ionic conduction in the YSZ-matrix and (iv) the gas diffusion in the pores (fig. 1a).
The complex anode electrochemistry strongly affects the performance and durability of SOFCs. For instance degradation phenomena like sulfur poisoning are known to hinder the electrooxidation of hydrogen due to chemisorption of sulfur on the catalytically active nickel surfaces. The dramatically lowered reaction rate of the electrochemical hydrogen oxidation is followed by a considerable extension of the penetration depth of all electrochemical subprocesses [3].
In this study a physical meaningful modelling approach based on a three channel transmission line model (TLM) was developed in order to describe the coupling of all abovementioned processes. The extension of the electrochemical active zone into the substrate is considered for the first time by investigating AFLs varying from 3 to 22 µm (fig. 1c). The model is capable of correlating the microstructure and thickness of AFL and AS with the performance of Ni/YSZ cermet anodes. The required model parameters were obtained from EIS measurements on patterned model anodes [4], 4-point DC conductivity measurements on electrolyte bulks and microstructure properties extracted from FIB-tomography for both layers [5]. We will present a detailed study on the development and parametrization of the TLM model, as well as the validation of the model with measured impedance data for different cell configurations. Evaluating the experimental data with the extended modelling approach gives new and significant insights into the complex electrochemistry of Ni/YSZ cermet anodes.
Anodes with thicker functional layer revealed (I) a poorer initial performance due to enhanced gas diffusion, whereas (II) the tolerance towards sulfur is increased. At 750°C the initial polarization of cells with 22 µm thick AFL is 47 mΩ∙cm² higher than for 7 µm and 27 mΩ∙cm² higher than for 3 µm, respectively. After sulfur exposure the 22 µm AFL cells show a 50 mΩ∙cm² smaller polarization than for 7 µm and even 129 mΩ∙cm² smaller polarization than for 3 µm as well as the least poisoning effect (+66%) of all measured cells (fig. 1d).
References:
[1] H. Schichlein, et. al., J. Appl. Electrochem., vol. 32(8), 875-882 (2002).
[2] A. Leonide, et. al., ECS Trans., vol. 19(20), 81-109 (2009).
[3] A. Kromp et al., J. Electrochem. Soc., vol. 159(5), p. B597 (2012).
[4] A. Utz, et. al., J. Electrochem. Soc., vol. 157(6), B920-B930 (2010).
[5] J. Joos, et. al., Proc. 10th EU SOFC Forum, A1006 (2012).
Figures:
Figure 1: a) Illustration of the reaction and transport processes in Ni/YSZ cermet anodes, b) SEM-image of the anode functional layer and substrate, c) modelling scheme of the TLM and d) measured EIS data at 750°C before and after sulfur exposure for different functional layer thicknesses.