Kinetic Studies on State of the Art Solid Oxide Cells

Electrochemical reaction kinetics at the electrodes of Solid Oxide Cells (SOCs) were investigated at 700 ◦ C for two cells with different fuel electrode microstructures as well as on a third cell with a reduced active electrode area. Three fuel mixtures were investigated – hydrogen/steam and model reformate fuels–hydrogen/carbon-dioxide and hydrogen/methane/steam. It was found that the electrode kinetics at the fuel electrode were exactly the same in both reformates. The hydrogen/steam fuel displayed 5–7% faster kinetics than the reformate fuels. 19% faster kinetics were recorded for the cell with a ﬁner microstructure. The measured gas conversion impedance was compared with models in literature for both the 16- and the 2 cm 2 cells. The continuously stirred tank reactor (CSTR) AC model approximated the overpotential of the smaller cells (2 cm 2 ) with greater accuracy in the current density range 0–0.5 A/cm 2 while the plug ﬂow reactor (PFR) model although derived for the case of zero DC bias predicted the 16 cm 2 cell ASR better than the zero bias CSTR model. Furthermore, the gas conversion impedance in the hydrogen/steam fuel split into two processes with opposing temperature behavior in the reformate fuels. By using a 87.5% smaller active electrode area the gas conversion impedance was diminished in the hydrogen/steam fuel at (the same absolute) high fuel ﬂow rates. In both reformates, the second and third lowest frequency processes merged into a single process as the gas conversion was reduced. The SOC with ﬁner


Introduction
The Solid Oxide Fuel Cell (SOFC), which converts hydrogen as well as hydrocarbon fuels directly into electricity, has demonstrated almost comparable performance when operated reversely as Solid Oxide Electrolyser Cell (SOEC) for electrical energy storage as fuels. In both applications of the technology, cell optimization and eventual commercialisation requires a sound understanding of the mechanisms that affect performance and stability. These mechanisms depend on operation conditions like temperature, gas composition, fuel utilisation and current load as well as on gradients along cell and stackand cell microstructure. This increases the complexity of the systems, such that deconvolution and analytical description of the involved mechanisms becomes a major challenge, especially if both macroscopic trends as well as fundamental chemistry are to be accommodated. The electrode reaction kinetics is one such mechanism.
The cells were characterised through electrochemical impedance spectroscopy at open circuit voltage and under current load. Current/voltage characteristics were recorded at different temperatures and gas compositions. The fuel utilization and current density range were chosen to emulate practical conditions. Figure 1 displays the Distribution of Relaxation Times (DRT) 1 of spectra recorded at varying current densities in SOFC mode at 700-and at 800 o C ensuring a maximum fuel utilization of 60% in three different fuel electrode gas mixtures. Considering that the area beneath a peak represents the Area Specific Resistance (ASR) of the given dynamic process, it is clear that depending on operation temperature and fuel composition the processes make varying contributions to the overall cell resistance.

Current vs. Overpotential Relation
The ASRs of the processes were obtained through Complex Non-linear Least Squares fit of the spectra with an equivalent circuit model 2 . The DRT was used for pre-identification of initial fit parameters.
By integration of the ASRs vs current density, the corresponding overpotential curves were obtained, as displayed in Figure 2 for the electrochemical oxidation of H 2 at the Ni/YSZ electrode. (f > 10 3 Hz in Figure 1) Figure 2 reveals that the overpotential associated with the electrochemical oxidation of H 2 2 (and CO) at the Ni/YSZ electrode at all current densities increases from H 2 /H 2 O to H 2 /CO 2 and CH 4 /H 2 /H 2 O fuel compositions. This is probably because in the reformate cases, H 2 has to be replenished through the water gas shift reaction 2 and also at equilibrium the H 2 /H 2 O case contains more H 2 O. The cooling effect of the endothermic steam reforming may contribute to a higher overpotential in CH 4 /H 2 /H 2 O mixture. Relative to the overall potential drop in the investigated current density range, it can be claimed though that under these conditions and within the limits of accuracy, the Ni/YSZ electrode displays comparable performance in the three different fuel gas compositions.

Outlook
In this work experimental results of investigations on cells with varying microstructure of the fuel electrode operated in H 2 /H 2 O and reformate mixtures will be presented. Detailed impedance analysis will be used to determine the current/overpotential relationships and corresponding partial pressure dependencies. This will provide a solid basis for discussion of the highly debated analytical description of the electrochemical reaction overpotential of the Ni/YSZ electrodes.