Reactions and Transport Pathways in Syngas Fueled Ni/YSZ SOFC Anodes: Experiments and Modeling

The fuel flexibility of solid oxide fuel cells (SOFC) is highly attractive for numerous technical applications that convert natural gas, kerosene or diesel reformate into electrical energy. For the operation with hydrocarbon fuels, there is a great deal of knowledge about the catalytic reforming chemistry in the SOFC anodes, whereas little is known about the solid/gas electrochemistry. This work reports a comprehensive understanding of the reaction mechanisms and transport pathways that determine the performance of syngas fueled Ni/YSZ anodes.

Based on our initial work, in which the individual loss mechanisms for syngas fueled Ni/YSZ anodes were identified via electrochemical impedance spectroscopy (EIS) [1], a schematic model of the reaction mechanisms and transport pathways is developed (Fig. 1). Since exclusively H₂ is electrochemically oxidized at the triple-phase boundary, the CO within the fuel is subsequently oxidized via the water-gas shift reaction [1]. The resulting gas transport properties within the anode substrate feature two transport pathways (H₂/H₂O and CO/CO₂), which are coupled by the water-gas shift reaction.

This schematic model is implemented in a transient finite element method (FEM) simulation [2]. The isothermal model represents the electrochemical fuel oxidation and the heterogeneous reforming chemistry on the catalytically active Ni-sites in terms of global kinetics. The multi component gas transport of the fuel gas species through the porous electrode structure is represented by the Maxwell-Stefan equation. Coherently calculating the complex species transport phenomena and the kinetics of the reforming chemistry, the model is capable to reproduce the multiple semicircles of the measured gas transport impedance. With the help of the FEM model, the characteristics of the measured impedance are explained in detail.

With the validation of the presented reaction diffusion network, an overall understanding of the loss mechanisms for syngas fueled Ni/YSZ anodes has been reported for the first time in literature [3]. With this knowledge, the impact of sulfur-poisoning on (i) the electrochemical fuel oxidation and on (ii) the heterogeneous reforming chemistry within syngas fueled Ni/YSZ anodes can be monitored separately [4].

References
[1] A. Kromp, A. Leonide, A. Weber, E. Ivers-Tiffée, J. Electrochem. Soc., 158 (8), B980-B986 (2011).
[2] A. Kromp, H. Geisler, A. Weber, E. Ivers-Tiffée, Electrochim. Acta 106, 418-424 (2013).
[3] A. Kromp, A. Weber, E. Ivers-Tiffée, ECS. Trans. 57 (1), 3063-3075 (2013).
[4] A. Kromp, S. Dierickx, A. Leonide, A. Weber, E. Ivers-Tiffée, J. Electrochem. Soc., 159 (5), B597-B601 (2012).