It has been reported for SOFC systems that high electrical efficiency of more than 70-80% could be achieved theoretically in a recent study [1]. However, their electrochemical performance would become lower due to an increase in ohmic and nonohmic resistances and possible fuel starvation. From these reasons, it is desired to develop a simulation technique for visualizing various phenomena in fuel cells. Here, we focus on numerical analysis on electrode kinetics. Since exchange current density is an important phenomenological parameter in considering electrochemical reaction kinetics at an SOFC electrode and in simulating the performance, the exchange current density of methane-fueled pre-reformed SOFC anodes was measured and compared with that of hydrogen-based fuel. Furthermore, the spatial distribution of current density was simulated for given average current density in case anode exchange current density i₀ is either constant or variable.

Experiment

In this study, electrolyte-supported cells Ni-ScSZ/ScSZ/LSM-ScSZ (ScSZ:10mol%Sc₂O₃- 1mol%CeO₂-89mol% ZrO₂) were used. The electrode area was 8×8mm² for i₀ measurements. Pt mesh was used as a current collector. In this study, by changing operation temperature, fuel utilization (Uf), and steam-to-carbon ratio (S/C), anode overvoltage was measured at each (average) current density (0.25, 0.3, 0.35, and 0.4 cm²) for methane-fueled pre-reformed SOFCs by the current interrupt method. The anode exchange current density was then derived from the current density and anode overvoltage values.

Results and discussion

Anode exchange current density for hydrogen-fueled SOFCs may be described by Eq. (I) after Hosoi et al. [2]. Anode exchange current density could also be described by Eq. (II) in case vacant surface adsorption sites are predominant [2,3]. Figure 1(a) shows experimental values derived for the pre-reformed fuel in this study. The values derived from Eq. (i) are also shown with solid lines in Fig. 1(a) and (b), while the values derived from Eq. (II) are shown with dashed lines in Fig. 1(b). All these figures indicate gradual decrease in anode exchange current density in the high fuel utilization (Uf) region.

i_0,a = 3.5・exp(-6.2×10⁴[Jmol^-1] / RT)・(P_H2 / P_H2,ref)^0.41・(P_H2O/P_H2O,ref)^0.4 Eq. (I)

i_0,a = a^’・exp(b^’ [Jmol^-1] /RT)・P_H2^0.5 ・P_H2O^0.5 Eq. (II)

3D-numerical simulation has been made using the software COMSOL to derive 2D current density of the cell shown in Fig. 1(c). Current density distribution for various average current densities is derived as a function of Uf and shown in Figs. 1(d) and 1(e) for constant i₀ and variable i₀, respectively. While Fig. 1(d) suggests continuous decrease in current density from inlet to outlet, Fig. 1(e) suggests almost constant current density near the inlet and its gradual decrease near the outlet. The tendency of the latter is rather consistent with the dependence of anode exchange current density as a function of Uf in Figs. 1(a) and 1(b) for hydrogen-based fuels.